The invention relates to parallel computational networks and circuit elements for such networks. The invention can be used in several applications in which variable values are connected by logical rules, including but not limited to applications in which the object is to perform reverse calculations, ie, calculations in which a result is known but the object is to find a set of starting values that gives the known result. There are several mathematical or logical operations which are grossly asymmetrical, which means that it is a straightforward task to carry out the operation in one direction but no fast algorithms are known or even supposed to exist for the reverse calculation.